High yield ratio hot rolled steel sheet which has excellent low temperature impact energy absorption and haz softening resistance and method of production of same

ABSTRACT

Hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has an excellent low temperature impact energy absorption and HAZ softening resistance and a method of production of the same are provided, that is, sheet which contains, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less, where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, has a balance of Fe and unavoidable impurities, has an area percentage of pearlite of 5% or less, has a total area percentage of martensite and retained austenite of 0.5% or less, has a balance of a metal structure of ferrite and/or bainite, has an average grain size of ferrite and bainite of 10 μm or less, has an average particle size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, and has a yield ratio of 0.85 or more.

TECHNICAL FIELD

The present invention relates to maximum tensile strength 600 MPa or more high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ (heat affected zone) softening resistance and a method of production of the same. The steel sheet is suitable as a base material for booms and frames of construction machinery and as a base material for frames, members, etc. of trucks and cars which are shaped mainly by bending and, further, as a base material for line pipe.

BACKGROUND ART

The frames of construction machinery and trucks are assembled by shaping hot rolled steel sheet mainly by bending and arc welding the shaped parts. Therefore, the base material which is used for these parts is required to have excellent bendability and arc weldability. Further, construction machinery and trucks are sometimes used in low temperature environments, so in particular with frames for trucks etc., the properties of being resistant to brittle fracture and of being able to sufficiently absorb impact energy when impact is given, even at a low temperature, are sought.

As steel sheet which has an excellent impact energy absorption, there is the art disclosed in NPLT 1 and PLTs 1 to 2. However, these steel sheets contain structures which include retained austenite or martensite and further optimize the metal structures of the steel sheets to achieve excellent impact properties. However, such structures of steel sheet had the problems of being low in yield stress and having issues in bendability.

Further, PLT 3 discloses a method of producing thin-gauge steel sheet which has a high impact energy absorption ability at a high yield in a stable manner by cold rolling. However, this method suffered from large softening of the heat affected zone (HAZ) of the arc weld zone and the inability to obtain a sufficient weld joint strength and, further, was disadvantageous in terms of production costs.

As a method of obtaining hot rolled steel sheet which has excellent bendability and a high yield ratio, for example, the method of dispersing Ti, Nb, and other alloy carbides in the steel such as shown in PLTs 4 to 6 has been disclosed. However, steel sheet which utilizes such precipitation strengthening sometimes suffers from a large softening of the arc weld heat affected zone and a drop in joint strength. Further, there were the problems that sometimes brittle fracture occurred at a low temperature and sometimes the amount of impact energy absorption became small.

On the other hand, as art to suppress softening of the weld heat affected zone, PLT 7 discloses the method of compositely adding Mo and Nb or Ti, while PLT 8 discloses the method of optimizing the ingredients so as to suppress HAZ softening even in precipitation strengthened steel which contains Ti. However, with these methods, there were the problems that sometimes brittle fracture occurred at a low temperature and sometimes the impact energy absorption amount became small.

PLT 9 discloses the method of establishing suitable rolling conditions from the rough rolling to finish rolling of the steel slab and a suitable subsequent cooling treatment so as to produce hot rolled steel sheet for high strength electric resistance welded steel pipe use which has excellent low temperature toughness and weldability. This method controls the recrystallization in the rough rolling and finish rolling of the steel slab to obtain a fine grain metal structure and obtain steel sheet which has excellent low temperature toughness, but does not intend control of the size or distribution of alloy carbonitrides. As a result, these are not optimized, so there was the problem of a drop in the impact energy absorption.

PLT 10 discloses a method of establishing a suitable rolling reduction rate and holding time in the rough rolling process of a steel slab and suitable finish rolling conditions so as to produce hot rolled high strength steel sheet which has excellent toughness and hydrogen cracking resistance. The object of the optimization of the rough rolling process in this method is the promotion of the recrystallization of steel, but this does not intend control of the size or distribution of alloy precipitates. As a result, these are not optimized, so there was the problem of a drop in the impact energy absorption. Regarding the finish rolling conditions as well, with the method described in PLT 10, there was the problem that it is not possible to control the size or distribution of the alloy precipitates and excellent impact absorption energy cannot be obtained.

PLT 11 discloses the art of suitably dispersing precipitated particles in the weld heat affected zone so as to obtain high strength hot rolled steel sheet which has an excellent HAZ softening resistance. However, this art disperses fine precipitates in the HAZ of the steel sheet during arc welding, but the size of the precipitated particles in the steel is not optimized, so as a result there was the problem that the steel sheet was not excellent in impact energy absorption.

CITATIONS LIST Patent Literature

-   PLT 1: Japanese Patent Publication No. 2007-284776A -   PLT 2: Japanese Patent Publication No. 2005-290396A -   PLT 3: Japanese Patent Publication No. 10-58004A -   PLT 4: Japanese Patent Publication No. 2009-185361A -   PLT 5: Japanese Patent Publication No. 2007-9322A -   PLT 6: Japanese Patent Publication No. 2005-264239A -   PLT 7: Japanese Patent Publication No. 2003-231941A -   PLT 8: Japanese Patent Publication No. 2001-89816A -   PLT 9: Japanese Patent Publication No. 2001-207220A -   PLT 10: Japanese Patent Publication No. 10-298645A -   PLT 11: Japanese Patent Publication No. 2008-280552A

Non-Patent Literature

-   NPLT 1: Nippon Steel Technical Reports, vol. 378 (2003), p. 2

SUMMARY OF INVENTION Technical Problem

The present invention was made in consideration of the above problems and has as its object the provision of maximum tensile strength 600 MPa or more high yield ratio hot rolled steel sheet which has both an excellent low temperature impact energy absorption and HAZ softening resistance and a method of production of the same.

Solution to Problem

The inventors etc. investigated in detail the factors influencing the HAZ softening and low temperature impact energy absorption of steel sheet which contains Ti and other alloy carbonitrides by which a high yield ratio can be stably obtained. As a result, they discovered that the amount of HAZ softening can be suppressed by establishing suitable amounts of Ti, Nb, and Mn.

Further, the inventors etc. next intensively studied the method of improving the low temperature impact energy absorption and discovered for the first time that by reducing the area percentage of pearlite in the metal structure of the steel sheet and rather eliminating as much as possible the retained austenite and martensite, which in the past had been considered advantageous for improvement of the impact energy absorption ability, and, further, by optimizing the lattice matching with the matrix Fe and size of the alloy carbonitrides which contain Ti and Nb which are dispersed in the steel, in particular controlling the particle size of alloy carbonitrides with incoherent interfaces, the low temperature impact energy absorption, which was an issue in precipitation strengthened steel, is improved.

In general, in precipitation strengthened steel which contains Nb and Ti, the precipitates are controlled so as to be present in a state of good lattice matching having a specific crystal orientation in relation to the matrix Fe, but this time the inventors etc. investigated the relationship with the low temperature impact energy absorption and as a result discovered that alloy carbonitrides in the precipitated state with good lattice matching with the matrix Fe tend not to become obstacles to starting and propagation of cracks, while alloy carbonitrides in an incoherent state with the matrix Fe lower the low temperature impact energy absorption amount even if relatively small in size. The mechanism by which lattice matching of the alloy carbonitrides with the matrix affects the low temperature impact energy absorption amount is not certain, but it may be that if the lattice matching of alloy carbonitrides and the matrix Fe is poor, this becomes a starting point for interfacial peeling or formation of voids and promotes both ductile fracture and brittle fracture.

The inventors etc. engaged in extensive studies on the process of production and ranges of ingredients for realizing the above type of structure and as a result completed maximum tensile strength 600 MPa or more hot rolled steel sheet and plated steel sheet which achieve both an HAZ softening resistance and low temperature energy absorption and further are high in yield ratio and excellent in bendability. That is, the gist of the present invention is as follows:

(1) High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, by mass %,

-   C: 0.04 to 0.09%, -   Si: 0.4% or less, -   Mn: 1.2 to 2.0%, -   P: 0.1% or less, -   S: 0.02% or less, -   Al: 1.0% or less, -   Nb: 0.02 to 0.09%, -   Ti: 0.02 to 0.07%, and -   N: 0.005% or less, -   a balance of Fe and unavoidable impurities, -   where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, and -   having a metal structure which comprises an area percentage of     pearlite of 5% or less, a total area percentage of martensite and     retained austenite of 0.5% or less, and a balance of one or both of     ferrite and bainite, -   having an average grain size of ferrite and bainite of 10 μm or     less, -   having an average grain size of alloy carbonitrides with incoherent     interfaces which contain Ti and Nb of 20 nm or less, -   having a yield ratio of 0.85 or more, and -   having a maximum tensile strength of 600 MPa or more.

(2) The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to (1), characterized by further comprising, by mass %, V: 0.01 to 0.12%.

(3) The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to claim 1 or 2, characterized by further comprising, by mass %, one or more of Cr, Cu, Ni, and Mo in a total of 0.02 to 2.0%.

(4) The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of (1) to (3), characterized by further comprising, by mass %, B: 0.0003 to 0.005%.

(5) The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of (1) to (4), characterized by further comprising, by mass%, one or more of Ca, Mg, La, and Ce in a total of 0.0003 to 0.01%.

(6) High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized that the high yield ratio hot rolled steel sheet according to any one of (1) to (5) is plated or alloy plated on a surface.

(7) A method of production of high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, heating a steel slab having a composition according to any one of (1) to (5) to 1150° C. or more, rough rolling the heated steel slab, finishing the rough rolling at temperature between 1000° C. to 1080° C., wherein a maximum rolling interval in the rough rolling which is performed at 1150° C. or less is 45 sec or less, after the rough rolling, holding the steel slab for a holding time t1 (sec) which satisfies the following formula (1), then starting finish rolling, performing finish rolling with a final rolling temperature Tf which satisfies the following formula (2) so as to obtain as steel sheet, starting water cooling of the steel sheet within 3 seconds after the finish rolling, then cooling the steel sheet to temperature 700° C. or less at a lowest cooling rate of 8° C./sec or more, and coiling the steel sheet at temperature between 530° C. to 650° C.

1000×([% Ti]+[% Nb])>t1   formula (1)

Tf>830+400([% Ti]+[% Nb])   formula (2)

(8) The method of production of high yield ratio hot rolled steel sheet according to (7) characterized in that a final rolling temperature Tf satisfies the following formula (3).

Tf>830+800([% Ti]+[% Nb])   formula (3)

(9) A method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, pickling the hot rolled steel sheet which was obtained by the method of production according to (7) or (8), heating steel sheet at the Ac3 temperature or less, then dipping the steel sheet in a plating bath to plate the surface of the steel sheet.

(10) The method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to (9) characterized by further comprising alloying the plated steel sheet after the plating.

Advantageous Effects of Invention

According to the hot rolled steel sheet of the present invention, due to the above configuration, it is possible to obtain high yield ratio hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has excellent HAZ softening resistance and low temperature energy absorption and further bendability. With conventional steel sheet, there were the problems that there were restrictions in use and operation at a low temperature and a sufficient joint strength could not be obtained, but according to the hot rolled steel sheet of the present invention, use in cold regions becomes possible, increased strength enables the products to be reduced in thickness, and the effect of reduction of weight of construction machinery, automobiles, and trucks can be expected.

Further, according to the method of production of hot rolled steel sheet which has an excellent low temperature impact energy absorption and a HAZ softening resistance of the present invention, it becomes possible to produce high yield ratio hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has excellent HAZ softening resistance and low temperature energy absorption and further bendability.

Note that, in the present invention, excellent low temperature impact energy absorption means the impact energy absorption in a Charpy impact test at −40° C. is 70 J/cm² or more. Further, excellent HAZ softening resistance means a difference ΔHV (=HV_(BM)−HV_(HAZ)) of 40 or less between the Vicker's hardness (HV_(HAZ)) of the softest part of the weld heat affected zone (HAZ) and the Vicker's hardness (HV_(BM)) of the base material at the time of arc welding by a weld current, voltage, and welding speed selected to give good bead shape and by a weld heat input of 10000 J/cm or less. Further, “excellent bendability” means an r_(lim)/t of 1.0 or less when the thickness of the test piece in a 90° V bending test is “t” and the limit radius of curvature where no cracks occur is r_(lim).

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] A graph which expresses the relationship between Mn+8Ti+12Nb and vE⁻⁴¹ and ΔHV.

[FIG. 2] A graph which expresses the effect of the amount of Ti+Nb on the relationship between the holding time t1 and vE⁻⁴⁰ from the final rough rolling to the start of the finish rolling.

[FIG. 3] A graph which expresses the relationship of the mass of Ti+Nb and Tf (° C.) of the invention examples and two types of comparative examples (A-7 and B-6) among the types of steel which are shown in Table 2.

DESCRIPTION OF EMBODIMENTS

Below, the present invention will be explained in detail. First, the reasons for limiting the steel ingredients of the high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance of the present invention will be explained. Here, the “%” for the ingredients means mass %.

“C: 0.04 to 0.09%”

If the amount of C is less than 0.04%, it is difficult to secure a maximum tensile strength of 600 MPa or more. On the other hand, if over 0.09%, the coarse and alloy carbonitrides with incoherent interfaces which contain Ti and Nb increase and the low temperature impact energy absorption falls, so the content was limited to 0.04% to 0.09% in range.

“Si: 0.4% or Less”

If the amount of Si exceeds 0.4%, sometimes martensite or retained austenite remains in the steel sheet structure and the low temperature toughness and impact energy absorption fall. For this reason, the suitable range was made 0.4% or less. From the viewpoint of securing the bendability, 0.2% or less is more preferable. The lower limit of the amount of Si is not particularly set, but if less than 0.001%, the production cost increases, so 0.001% is the substantive lower limit.

“Mn: 1.2 to 2.0%”

Mn is used to secure the strength of the matrix through control of the metal structure of the steel. Further, this is an element which contributes to the suppression of HAZ softening of the weld zone. If less than 1.2%, the area percentage of the pearlite increases, the low temperature impact energy absorption falls, and further the amount of HAZ softening increases, so the strength of the welded joint greatly falls compared with the strength of the matrix. If over 2.0% is contained, sometimes hard martensite is formed and the low temperature impact energy absorption falls, so the suitable range is made 2.0% or less. From the viewpoint of securing the bendability, the content is more preferably 1.8% or less.

“P: 0.1% or Less”

P is used for securing the strength of steel. However, if over 0.1% is included, the low temperature toughness falls and, further, the low temperature impact energy absorption cannot be obtained, so the suitable range is made 0.1% or less. The lower limit is not particularly set, but if less than 0.001%, the production cost increases, so 0.001% is the substantive lower limit.

“S: 0.02% or Less”

S is an element which affects the impact energy absorption. If over 0.02% is included, even if controlling the area percentage of the metal structure and the average particle size of the alloy carbonitrides, a low temperature impact energy absorption cannot be obtained, so the suitable range is made 0.02% or less. The lower limit is not particularly set, but if less than 0.0003%, the production cost increases, so 0.0003% is the substantive lower limit.

“Al: 1.0% or Less”

Al is used for deoxidation and control of the metal structure of the steel sheet. If over 1.0%, the heat affected zone in arc welding softens and a sufficient welded joint strength cannot be obtained, so the suitable range is made 1.0% or less. The lower limit is not particularly set, but if less than 0.001%, the production cost increases, so 0.001% is the substantive lower limit.

“Nb: 0.02 to 0.09%”

Nb is used as a precipitation strengthening element for adjusting the strength of the steel and is used for suppressing softening of the weld HAZ. If less than 0.02%, no effect of suppression of softening of the weld HAZ is seen, while if over 0.09%, coarse alloy carbonitrides which contain incoherent precipitated Ti and Nb increase and the low temperature impact energy absorption becomes lower, so the content was limited to 0.02% to 0.09% in range.

“Ti: 0.02 to 0.07%”

Ti is used as a precipitation strengthening element for adjusting the strength of the steel and is used for suppressing softening of the weld HAZ. If less than 0.02%, obtaining the maximum tensile strength of 600 MPa or more is difficult. Further, if over 0.07%, incoherent precipitated coarse alloy carbonitrides which contain Ti and Nb increase and the low temperature impact energy absorption becomes lower, so the content is limited to 0.02% to 0.07% in range. To stably obtain a yield ratio of 0.85 or more, 0.03% is preferably made the lower limit.

“N: 0.005% or Less”

N contributes to the grain size of the metal structure of the steel sheet through formation of nitrides. However, if over 0.005%, the coarse and alloy carbonitrides with incoherent interfaces which contain Ti and Nb increase and the low temperature impact energy absorption becomes lower, so the content was limited to 0.005% or less in range. The lower limit is not particularly set, but if less than 0.0003%, the production cost increases, so 0.0003% is the substantive lower limit.

“2.0≦Mn+8[% Ti]+12[% Nb]2.6”

“Mn+8[% Ti]+12[% Nb]” is the total of the ratios of contribution of the different elements relating to the low temperature impact energy absorption and the HZ softening due to welding. As shown in FIG. 1, if plotting the relationship of the indicator of impact energy absorption of vE⁻⁴⁰ and the indicator of HAZ softening of ΔHV for 11 types of steel differing in Ti and Nb, if the value of this parameter is less than 2.0, a sufficient HAZ softening resistance cannot be obtained (that is, ΔHV>40) and obtaining a maximum tensile strength of 600 MPa or more becomes difficult, while if over 2.6, the coarse and alloy carbonitrides with incoherent interfaces which contain Ti and Nb increase and the low temperature impact energy absorption becomes lower (that is, vE⁻⁴⁰<70 J/cm²). For this reason, the suitable range was limited to 2.0 to 2.6 in range.

In the present invention, as steel ingredients, in addition to the above essential elements, it is also possible to selectively include the following such elements.

“V: 0.01 to 0.12%”

V may be used to adjust the strength of the steel. However, if the content of V is less than 0.01%, there is no such effect. Further, if over 0.12%, embrittlement proceeds and the low temperature impact energy absorption falls. For this reason, the suitable range was limited to 0.01 to 0.12%.

“One or More of Cr, Cu, Ni, and Mo in Total of 0.02 to 2.0%”

Cr, Cu, Ni, and Mo may be used to control the structure of the steel. However, if the total content of the one or more of these elements is less than 0.02%, there is no above effect accompanying addition. Further, if over 2.0%, austenite is retained and the low temperature impact energy absorption falls. For this reason, the suitable range of the total of these elements was limited to 0.02 to 2.0%.

“B: 0.0003 to 0.005%”

B may be used for control of the structure of the steel sheet. However, if the amount of B is less than 0.0003%, that effect is not exhibited. Further, if over 0.005%, martensite is sometimes formed and the low temperature impact energy absorption falls. For this reason, the suitable range was limited to 0.0003 to 0.005%.

“One or More of Ca, Mg, La, and Ce in a Total of 0.0003 to 0.01%”

Ca, Mg, La, and Ce may be used for deoxidation of the steel. However, if the total amount of the one or more of these elements is less than 0.0003%, there is no such effect, while if over 0.01%, brittle fracture occurs at a low temperature and the impact energy absorption falls. For this reason, the suitable range was limited to 0.0003 to 0.01%.

Note that the balance of the ingredients is Fe and unavoidable impurities, but the steel ingredients in the present embodiment are not particularly limited in regard to other elements. Various elements may be suitably included for adjusting the strength.

Next, the metal structure of the hot rolled steel sheet of the present invention will be explained.

The hot rolled steel sheet of the present invention may contain ferrite and bainite as main phases and a balance of one or more of pearlite, martensite, and retained austenite.

“Area Percentage of Pearlite”

In precipitation strengthened steel which contains Nb and Ti, if the area percentage of pearlite exceeds 5%, brittle fracture easily occurs at a low temperature and, further, the impact energy absorption falls, so the upper limit was made 5%. From the viewpoint of securing the bendability, 3% or less is the preferable range. Note that, the lower limit is not particularly set, but having an area percentage of pearlite of close to zero is more preferable in regard to the impact energy absorption.

“Total Area Percentage of Martensite and Retained Austenite”

In precipitation strengthened steel which contains Nb and Ti, if the total area percentage of martensite and retained austenite exceeds 0.5%, brittle fracture easily occurs at a low temperature and, further, the impact energy absorption falls. For this reason, the upper limit of the total area percentage was made 0.5%. Note that, the lower limit is not particularly set, but having a total area percentage of martensite and retained austenite of close to zero is more preferable in regard to the impact energy absorption.

“Metal Structure which has Balance of One or Both of Ferrite and Bainite”

The area percentages of these are not particularly limited, but from the viewpoint of securing bendability, the bainite area percentage is preferably made 10% or more.

“Average Grain Size of Ferrite and Bainite”

The average grain size of ferrite and bainite is a correlative factor. If the average particle size is over 10 μm, even if controlling the average particle size of the alloy carbonitrides which contain Nb and Ti, sometimes the low temperature impact energy absorption cannot be secured, so the upper limit was made 10 μm. 8 μm or less is a preferable condition enabling impact energy absorption to be more stably secured. The lower limit is not particularly set, but if the size is less than 2 μm, the production cost greatly increases, so 2 μm is the substantive lower limit.

In the present invention, the metal structure of the steel sheet can be observed based on JIS G 0551 by an optical microscope. The observed surface is obtained by polishing the steel sheet, then etching it by a Nital corrosive solution.

The area percentages of ferrite, bainite, pearlite, and martensite can be measured by the point count method or image analysis using structural photographs obtained by an optical microscope or scan type electron microscope (SEM). The area percentage of retained austenite is measured by X-ray diffraction.

In the present invention, “bainite” includes upper bainite, lower bainite, and granular bainite. Further, “pearlite” includes pearlite and pseudo pearlite.

The grain size can be measured by observation by an optical microscope or by crystal orientation analysis by the EBSD method. Here, “the grain size” is the average grain size “d” which is described in JIS G 0551.

“Average Particle Size of Alloy Carbonitrides with Incoherent Interfaces which Contain Ti and Nb”

The particle size of alloy carbonitrides which contain Ti and Nb and the lattice matching with the matrix structure ferrite or bainite are important factors relating to the low temperature impact energy absorption. In general, in precipitation strengthened steel, it is known to cause the precipitation of fine alloy carbonitrides with good lattice matching with the matrix structure as fine particles, but for improvement of the low temperature toughness and improvement of the impact energy absorption, it is important to control the alloy carbonitride particles with poor lattice matching with the matrix structure. If the average particle size of the alloy carbonitrides with incoherent interfaces which degrade the lattice matching is over 20 nm, the low temperature impact energy absorption falls, so the suitable range was limited to 20 nm or less. From the viewpoint of obtaining a better impact energy absorption, 10 nm or less is the more preferable range. The lower limit is not particularly set, but as a size enabling analysis of the crystal orientation of the precipitate, 2 nm is the substantive lower limit.

Here, “alloy carbonitrides with incoherent interfaces” means the state not coherent precipitated in the matrix structure of ferrite or bainite and adjoining ferrite and bainite not having the following crystal orientation relationships (Baker-Nutting orientation relationships):

-   (100)MX//(100) Fe -   (010)MX//(011) Fe -   (001)MX//(0-11)Fe (Note: −1 is alternative notation for 1 with bar     above it)

Here, M indicates Ti and Nb. The percentages occupied by Ti and Nb are not an issue. Further, X indicates C and N. The percentages occupied by C and N are not an issue. When adding V or Mo, sometimes M contains V or Mo.

Note that, the alloy carbonitrides with incoherent interfaces were analyzed for crystal orientation and measured for average particle size using a transmission type electron microscope (TEM). First, a steel slab sample was rendered into a thin film of an extent through which electron beams pass, the TEM was used to analyze the crystal orientation between the precipitate and the surrounding matrix phase Fe, then the average particle size of 20 precipitates in order from the largest diameter precipitates in the precipitates which were judged to be incoherent precipitates was measured. Here, the “particle size of a precipitate” is measured as the equivalent circle diameter when assuming a circle equivalent to the cross-sectional area of a particle.

“Yield Ratio of 0.85 or More”

If the yield ratio is less than 0.85, sometimes the low temperature impact energy absorption falls and the bendability falls. For this reason, the lower limit of the yield ratio was made 0.85.

Note that, in the present invention, r_(lim)/t was used as the criteria for evaluation of the bendability. Here, “t” is the thickness of the test piece and r_(lim) is the limit radius of curvature at which no cracks occur in a 90° V-bending test. An r_(lim)/t of 1.0 or less was deemed good bendability. 0.5 or less is the more preferable range. The upper limit is not particularly set, but if the value is over 1.1, the bendability may fall, so 1.1 or less is the more preferable range.

“Maximum Tensile Strength of 600 MPa or More”

If the maximum tensile strength is less than 600 MPa, the steel sheet does not contribute to reduction of weight of the members of cars, trucks, construction machinery, etc., so in the present invention, steel sheet of a maximum tensile strength of 600 MPa or more is assumed.

Next, the method of production will be explained in detail.

Before the hot rolling, it is necessary to heat the steel slab of the ingredients which are prescribed in the present invention to 1150° C. or more to render the alloy carbonitrides which are present in the steel slab a solid solution state. If the heating temperature is less than 1150° C., it becomes difficult to obtain a strength of a maximum tensile strength 600 MPa or more. Further, the coarse alloy carbonitrides do not sufficiently dissolve and as a result coarse alloy carbonitrides remain, so the low temperature impact energy absorption falls. For this reason, the heating temperature of the steel slab was limited to 1150° C. or more. The upper limit is not particularly set, but if over 1300° C., the effect becomes saturated, so this is the substantive upper limit.

The above heated steel slab is rough rolled to a rough bar. This rough rolling has to be completed between 1000° C. to 1080° C. If the finishing temperature is less than 1000° C., coarse alloy carbonitrides precipitate in the austenite and the low temperature impact energy absorption falls, while if 1080° C. or more, the austenite grains become coarser, it is not possible to obtain an average grain size of ferrite and bainite of 10 μm or less in the transformed structure after finish rolling, cooling, and coiling, the low temperature toughness deteriorates, and the impact energy absorption falls. Further, in rough rolling performed at 1150° C. or less, the holding time between rolling reduction passes is an important parameter which affects the average particle size of the incoherent alloy carbonitrides. In the method of the present invention, the rough rolling is usually performed by rolling 3 to 10 times or so, more preferably rolling 5 to 10 times, but if the maximum holding time t0 between rolling passes performed at 1150° C. or less is 45 sec or more, the alloy carbonitrides become coarser to an extent affecting the impact energy absorption. For this reason, the holding time between rolling reduction passes was limited to within 45 seconds. Within 30 sec is more preferable.

Next, the rough bar is finish rolled to obtain a rolled material.

The time (t1) from after rough rolling finishes to the start of the finish rolling is an important parameter which affects the average particle size of the alloy carbonitrides and the grain size of the ferrite and bainite after transformation. As shown in FIG. 2, the greater the total amount of Ti and Nb, the more the holding time t1 (arrow mark in figure) where the impact energy absorption (vE⁻⁴⁰) shifts from good (OK) to no good (NG) increases. The holding time t1 (sec) where the absorption shifts from good (OK) to no good (NG) substantially corresponds to 1000×([% Ti]+[% Nb]). In this way, if the holding time t1 (sec) from after the rough rolling finishes to when the finish rolling starts is 1000×([% Ti]+[% Nb])sec or more, coarse alloy carbonitrides precipitate in the austenite, the austenite crystal grains become coarser, it is not possible to obtain an average grain size of ferrite and bainite of 10 μm or less in the transformed structure after the finish rolling, cooling, and coiling, the low temperature toughness deteriorates, and the impact energy absorption falls. 700×([% Ti]+[% Nb])>t1 sec is the more preferable range. Accordingly, the holding time t1 (sec) was defined by the following formula (1):

1000×([% Ti]+[% Nb])>t1   formula (1)

Further, in hot finish rolling, the final rolling temperature Tf has an effect on the average particle size of the alloy carbonitrides and the grain size of ferrite and bainite after transformation, so is an important condition in the present invention and changes depending on the contents of Ti and Nb.

It was learned that if the final rolling temperature Tf is 830+400×([% Ti]+[% Nb]) or less, coarse alloy carbonitrides with no lattice matching with the matrix precipitate and the low temperature impact energy absorption falls. Therefore, the final rolling temperature Tf is set so as to satisfy the following formula (2).

Tf>830+400([% Ti]+[% Nb])   formula (2)

This relationship (2) is found from the relationship of the type of steel of Table 2 explained later and the final rolling temperature Tf. FIG. 3 shows the relationship between the mass % of Ti+Nb and Tf (° C.) of an invention example and comparative example (A-7 and B-6) in the types of steel which are shown in Table 2. Here, it is learned that the case where the coefficient “a” of the part “a([% Ti]+[% Nb])” is made 400, that is, formula (2), is the boundary at which the −40° C. impact absorption energy vE⁻⁴⁰ becomes 70 J/cm² or more.

When the coefficient “a” is 800, that is, when

Tf>830+800([% Ti]+[% Nb])   formula (3),

compared with when the coefficient “a” is 400, the −40° C. impact absorption energy vE⁻⁴⁰ shifts somewhat from the boundary of 70 J/cm² or more. However, in the region where the coefficient “a” is 400 to 800, the wait time until the start of finish rolling becomes longer and the possibility of alloy carbonitrides starting to precipitate becomes higher, so the Tf is preferably controlled based on the formula (3) where the coefficient “a” is 800.

The upper limit of the final rolling temperature Tf is not particularly set, but the grain size of the ferrite and bainite tends to become coarser, so 970° C. or less is more preferable.

Right after the final rolling, the rolled material is water cooled. The time from when the final rolling finishes to the start of air cooling has an effect on the low temperature base material toughness and impact energy absorption through the y-particle size and average particle size of the alloy carbonitrides. If the air-cooling time right after the final rolling exceeds 3 sec, the impact energy absorption tends to fall, so the water cooling is started within 3 seconds. The lower limit is not particularly set, but in general facilities is substantially 0.2 sec or more.

After the air cooling right after the final rolling, the rolled material is cooled to obtain the hot rolled steel sheet. This cooling is an important process for controlling the metal structure. The cooling is performed down to 700° C. or less by the lowest cooling rate of 8° C. /sec or more.

If the stop temperature of the cooling exceeds 700° C., alloy carbonitrides easily precipitate coarsely at the grain boundaries, pearlite easily forms, the grain size of the ferrite becomes larger, and the low temperature impact energy absorption falls. On the other hand, when the lowest cooling rate down to 700° C. is less than 8° C./sec, the alloy carbonitrides easily precipitate coarsely at the grain boundaries, pearlite easily forms, the grain size of the ferrite becomes larger, and the low temperature impact energy absorption falls.

Here, a lowest cooling rate 8° C./sec or more means that the cooling rate between temperatures from the air-cooling finishing temperature to 700° C. never becomes lower than 8° C./sec. For this reason, for example, this means air cooling is not performed in this temperature range. In this way, in the present invention, air-cooling is not performed in the middle of the cooling process using water cooling unlike in the past.

The cooling stop temperature is more preferably 680° C. or less, while the lowest cooling rate is more preferably 15° C./sec or more. The upper limit of the lowest cooling rate is not particularly set, but if the rate is over 80° C./sec, uniform cooling in the hot rolled coil becomes difficult and the fluctuations in strength in the coil become greater. For this reason, 80° C./sec or less is preferable.

Next, the cooled hot rolled steel sheet is coiled up. The coiling temperature is made 530 to 650° C. If the coiling temperature is less than 530° C., sometimes martensite or retained austenite forms and the drop in low temperature toughness and drop in impact energy absorption become remarkable. Further, if over 650° C., the area percentage of the pearlite becomes greater and the drop in low temperature toughness and drop in impact energy absorption become remarkable.

The thus obtained hot rolled steel sheet may also be reheated (annealed). In this case, if the temperature of the reheating exceeds the Ac3 temperature, coarse alloy carbonitrides precipitate and the low temperature impact energy absorption falls. For this reason, the suitable range of the reheating temperature is limited to the Ac3 temperature or less. The heating method is not particularly designated and may be a method using furnace heating, induction heating, ohmic heating, high frequency heating, etc.

The heating time is not particularly determined, but if the heating and holding time at 550° C. or more exceeds 30 minutes, to obtain a 590 MPa or more tensile strength, the highest heating temperature is preferably made 700° C. or less.

Note that, the reheating (annealing) may be performed after coiling the hot rolled steel sheet and before the temperature falls to room temperature.

Skin pass rolling or leveler rolling is effective for correcting the shape, aging, and improving the fatigue characteristics, so may be performed after pickling or before pickling. If performing skin pass rolling, the upper limit of the rolling rate is preferably made 3%. This is because if over 3%, the shapeability of the steel sheet is impaired. Further, pickling may be performed in accordance with the objective.

Next, the hot dipped galvanized steel sheet and method of production of the same of the present invention will be explained.

The hot dipped galvanized steel sheet of the present invention is the above-mentioned hot rolled steel sheet of the present invention on the surface of which a plating layer or alloy plating layer is provided.

The hot rolled steel sheet which was obtained by the above-mentioned method was pickled, then a continuous galvanization facility or continuous annealing and galvanization facility was used to heat the steel sheet and hot dip coat it to form a plating layer on the surface of the hot rolled steel sheet.

If the heating temperature of the steel sheet exceeds the Ac3 temperature, a drop in the tensile strength of the steel sheet and a drop in the low temperature impact energy absorption occur, so the suitable range of the heating temperature is limited to the Ac3 temperature or less. The closer the heating temperature to Ac3, the more rapidly the tensile strength falls. The base materials greatly fluctuate in grade, so Ac3-30° C. or less is the more preferable range of heating temperature.

Further, after the hot dip coating, galvannealization may be performed to obtain a hot dip galvannealed layer.

Note that, the plating type is not limited to galvanization. It may also be other plating so long as the upper limit of the heating temperature is the Ac3b temperature.

Further, in the present invention, the method of production preceding the hot rolling is not particularly limited. That is, a blast furnace, converter, electric furnace, etc. may be used for melting, then various types of secondary refining may be used to adjust the ingredients to give the targeted contents of ingredients. Next, the steel may be cast by any method such as normal continuous casting, casting by the ingot method, or also thin slab casting etc. For the feed material, scrap may also be used. In the cast of a slab which is obtained by continuous casting, the high temperature cast slab may be directly sent as is to the hot rolling mill or may be cooled down to room temperature, then reheated at a heating furnace and then hot rolled.

EXAMPLES

Below, examples will be used to further explain the present invention.

Steels A to AC which have the chemical ingredients which are shown in Table 1 were produced by the following method. First, the steels were cast to prepare steel slabs, then the steel slabs were reheated and rough rolled to rough bars under the hot rolling conditions and annealing and plating conditions which are shown in Table 2-1 and Table 2-2. Next, the rough bars were finish rolled to obtain 4 mm thick rolled materials, then these were cooled and taken up as hot rolled steel sheet.

TABLE 1 Steel No. C Si Mn P S Al Ti Nb N Mn + 8Ti + 12Nb Ac3 Others Remarks A 0.04 0.3 1.7 0.01 0.001 0.05 0.03 0.05 0.002 2.5 853 Inv. steel B 0.05 0.3 1.5 0.01 0.001 0.8 0.07 0.04 0.003 2.5 900 Inv. steel C 0.08 0.03 1.2 0.02 0.002 0.03 0.06 0.04 0.003 2.2 857 Inv. steel D 0.06 0.03 1.4 0.01 0.003 0.03 0.05 0.04 0.002 2.3 848 Ca: 0.0015 Inv. steel E 0.04 0.3 1.8 0.01 0.003 0.03 0.06 0.05 0.003 2.9 861 Comp. steel F 0.09 0.03 1.3 0.01 0.005 0.03 0.03 0.02 0.002 1.8 832 Comp. steel G 0.02 0.03 1.5 0.01 0.003 0.04 0.05 0.03 0.002 2.3 866 Comp. steel H 0.10 0.03 1.3 0.01 0.003 0.04 0.03 0.04 0.002 2.0 829 Comp. steel I 0.05 0.5 1.3 0.01 0.003 0.04 0.03 0.04 0.002 2.0 869 Comp. steel J 0.05 0.03 1.0 0.01 0.003 0.04 0.03 0.07 0.003 2.1 857 Comp. steel K 0.05 0.03 2.1 0.01 0.003 0.04 0.04 0.04 0.003 2.9 828 Comp. steel L 0.05 0.03 1.3 0.08 0.003 0.04 0.04 0.04 0.003 2.1 901 Inv. steel M 0.05 0.03 1.3 0.12 0.003 0.04 0.04 0.04 0.003 21 929 Comp. steel N 0.05 0.03 1.3 0.01 0.015 0.04 0.04 0.04 0.003 2.1 852 Inv. steel O 0.05 0.03 1.3 0.01 0.022 0.04 0.04 0.04 0.003 2.1 852 Comp. steel P 0.05 0.03 1.3 0.01 0.003 1.3 0.04 0.04 0.003 2.1 902 Comp. steel Q 0.05 0.03 1.3 0.01 0.003 0.04 0.005 0.05 0.003 1.9 838 Comp. steel R 0.05 0.03 1.3 0.01 0.003 0.04 0.09 0.06 0.003 2.7 872 Comp. steel S 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.003 0.003 1.7 852 Comp. steel T 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.10 0.003 2.8 852 Comp. steel U 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.006 2.1 852 Comp. steel V 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.003 2.1 858 V: 0.06 Inv. steel W 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.003 2.1 848 Cr: 0.3, Cu: 0.05, Ni: 0.05 Inv. steel X 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.003 2.1 851 Mo: 0.3, B: 0.002 Inv. steel Y 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.003 2.1 852 Ce: 0.002, La: 0.001 Inv. steel Z 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04 0.003 2.1 842 Mg: 0.002, Cu: 0.5 Inv. steel AA 0.04 0.3 1.9 0.01 0.001 0.05 0.02 0.02 0.002 2.3 842 Inv. steel AB 0.04 0.3 2.1 0.01 0.001 0.05 0.02 0.02 0.002 2.5 836 Comp. steel AC 0.04 0.3 1.8 0.01 0.001 0.05 0.01 0.003 0.002 1.9 841 Comp. steel

TABLE 2 SRT RFT t0 t1 Tf t2 CRmin SCT CT Max. annealing (° C.) (° C.) (sec) (sec) (° C.) (sec) (° C./s) (° C.) (° C.) temp. (° C.) Plating type Remarks A-1 1230 1020 25 50 900 2 25 680 600 Inv. ex. A-2 1130 1000 25 50 900 2 25 680 600 A-3 1230 960 25 50 900 2 25 680 600 A-4 1230 1100 25 50 900 2 25 680 600 A-5 1230 1020 25 20 900 2 25 680 600 Inv. ex. A-B 1230 1020 25 120 900 2 25 680 600 A-7 1230 1020 25 50 860 2 25 680 600 A-8 1230 1020 25 50 900 6 12 680 600 A-9 1230 1020 25 50 900 2 15 700 640 Inv. ex. A-10 1230 1020 25 50 900 2 5 680 600 A-11 1230 1020 25 50 900 2 20 720 680 A-12 1230 1020 25 50 900 2 25 560 520 A-13 1230 1020 25 50 900 2 25 610 550 680 Galvanization Inv. ex. A-14 1230 1020 25 50 900 2 30 580 530 680 Galvannealization Inv. ex. A-15 1230 1020 25 50 900 2 25 680 600 880 Galvanization A-16 1230 1020 50 50 900 2 25 680 600 A-17 1230 1020 70 50 900 2 25 680 600 A-18 1230 1020 120 50 900 2 25 680 600 B-1 1250 1040 25 60 880 2 50 650 570 Inv. ex. B-2 1250 1000 25 60 880 2 50 650 570 Inv. ex. B-3 1250 970 25 120 880 2 50 650 570 B-4 1250 1100 25 60 880 2 50 650 570 B-5 1250 1040 25 60 880 2 50 650 570 Inv. ex. B-6 1250 1040 25 60 850 2 50 650 570 B-7 1250 1040 25 60 880 6 10 650 570 B-8 1250 1040 25 60 880 2 5 650 570 B-9 1250 1040 25 60 880 2 50 680 620 Inv. ex. B-10 1250 1040 25 60 880 2 50 710 660 B-11 1250 1040 25 60 880 2 50 510 480 B-12 1250 1040 50 60 880 2 50 650 570 B-13 1250 1040 120 60 880 2 50 650 570 C-1 1250 1040 25 45 880 2 50 570 600 Inv. ex. C-2 1250 1040 25 45 880 2 50 670 600 730 Galvanization Inv. ex. D-1 1259 1040 25 45 889 2 50 670 600 Inv. ex. E-1 1250 1040 25 60 880 2 50 670 600 F-1 1250 1040 25 60 880 2 50 670 600 G-1 1250 1040 25 60 880 2 50 670 600 H-1 1250 1040 25 60 880 2 50 679 600 I-1 1250 1040 25 60 880 2 50 670 600 J-1 1250 1040 25 60 880 2 50 670 600 K-1 1250 1040 25 60 880 2 50 670 600 L-1 1250 1040 25 45 880 2 50 670 600 Inv. ex. M-1 1250 1040 25 45 880 2 50 670 600 N-1 1250 1040 25 45 880 2 50 670 600 Inv. ex. O-1 1250 1040 25 60 880 2 50 670 600 P-1 1250 1040 25 60 880 2 50 670 600 Q-1 1250 1040 25 60 880 2 50 670 600 R-1 1250 1040 25 60 889 2 50 670 600 S-1 1250 1040 25 60 880 2 50 670 600 T-1 1250 1040 25 60 880 2 50 670 600 U-1 1250 1040 25 60 880 2 50 670 600 V-1 1250 1040 25 50 880 2 50 670 600 Inv. ex. W-1 1250 1040 25 50 880 2 50 670 600 Inv. ex. X-1 1250 1040 25 50 880 2 50 670 600 Inv. ex. Y-1 1250 1040 25 50 880 2 50 670 600 Inv. ex. Z-1 1250 1040 25 50 880 2 50 670 600 Inv. ex. AA-1 1250 1040 25 50 860 2 50 670 600 Inv. ex. AB-1 1250 1040 25 50 889 2 50 670 600 AC-1 1250 1040 25 50 880 2 50 670 600 SRT: Slab heating temperature RFT: Rough rolling finishing temperature t0: Rolling time at rough rolling performed at 1150° C. or less t1: Time from end of rough rolling to start of finish rolling Tf: Final finish rolling temperature t2: Air cooling time after final finish rolling CRmin: Minimum cooling rate during CFT from after air cooling SCT: Water cooling stop temperature CT: Coiling temperature

In Table 1, the chemical compositions are given by mass %. Further, in Table 1, Ac3(° C.) is the value which is calculated by the following formula:

Ac3=910−210√[% C]+45[% Si]−30[% Mn]+700[% P]+40[% Al]+400[% Ti]+32[% Mo]−11[% Cr]−20[% Cu]−15[% Ni]

wherein, % C, % Si, % Mn, % P, % Al, % Ti, % Mo, % Cr, % Cu, and % Ni respectively indicate the contents in steel of C, Si, Mn, P, Al, Ti, Mo, Cr, Cu, and Ni.

In Table 1, the chemical compositions of the steels correspond to the chemical compositions of the steels of the steel numbers in Table 2 with the same alphabet letters as the steel numbers.

In Table 2, “SRT” indicates the slab reheating temperature (° C.). “RFT” indicates the rough rolling finish temperature (° C.). “t0” indicates the maximum holding time (sec) between rough rolling operations performed at 1150° C. or less. “t1” indicates the time (sec) from the end of the rough rolling to the start of the finish rolling. “Tf” indicates the final finish rolling temperature (° C.). “t2” shows the air cooling time right after the last finish rolling (sec). “CRmin” indicates the minimum cooling rate in the SCT after air cooling (° C./sec). “SCT” indicates the water cooling stop temperature (° C.). “CT” indicates the coiling temperature (° C.).

The Steels A-12 to A-14 and C-2 are hot dipped galvanized steel sheets which were produced by pickling the hot rolled steel sheets, then annealing them on a continuous annealing and galvanization line at the annealing temperatures which are shown in Table 2, then galvanizing them.

Note that, the galvanization dipping temperature was made 450° C. while, for galvannealing treatment, the alloying temperature was made 500° C.

First, the metal structures and alloy carbonitrides of the prepared steel sheet were examined.

The metal structure of the steel sheet, as explained above, was observed based on JIS G 0551 for the L-cross-section by an optical microscope. Further, the area percentages of the different structures were measured by the point count method or image analysis using structural photographs at regions of ¼ t thickness of the L-cross-section (position of ¼ t from surface of steel sheet when sheet thickness is “t”). The grain sizes of the ferrite and bainite were measured by calculating the nominal particle size based on JIS G 0552.

The alloy carbonitrides with incoherent interfaces which contain Ti and NB were analyzed for crystal orientation and measured for average particle size by rendering the steel slab sample into a thin film of an extent through which electron beams pass and using a transmission type electron microscope (TEM). 20 or more alloy carbonitride particles were examined.

Next, to measure the amount of softening of the weld heat affected zone (HAZ), arc welding was used to prepared a lap joint. The welding was performed in an atmosphere of CO₂: 100% with a heat input of about 5000 to 8000 J/cm in range. After welding, the cross-section was polished and the base material and the weld heat affected zone (HAZ) were tested for Vicker's hardness aiming at 0 or less softening. The above measurement results are shown in Table 3. Note that, in Table 3, “F” indicates ferrite, “B” indicates bainite, “A” indicates retained austenite, “M” indicates martensite, and “P” indicates pearlite, “d_((F, B))” indicates the average grain size (μm) of ferrite and bainite, “d_(MCN)” indicates the average particle size (nm) of alloy carbonitrides with incoherent interfaces, and “ΔHV” indicates the difference between HV_(BM) and HV_(HAZ) when the Vicker's hardness of the softest part of the weld heat affected zone is HV_(HAZ) and the Vicker's hardness of the base material is HV_(BM).

TABLE 3 Metal structure YP TS percentage (%) Steel No. (MPa) (MPa) El (%) YR F + B M A P d_((F,B)) d_(MCN) ΔHV vE₋₄₀ Bendability Remarks A-1 600 640 25 0.94 98 2 8 12 20 120 VG Inv. ex. A-2 550 590 27 0.93 98 2 9 25 18 60 VG A-3 590 530 25 0.94 98 2 8 22 20 60 VG A-4 600 645 25 0.93 98 2 14 14 18 55 VG A-5 600 640 25 0.94 98 2 7 13 19 110 VG Inv. ex. A-6 600 640 25 0.94 100 12 18 20 60 VG A-7 590 630 25 0.94 94 6 8 21 22 65 VG A-8 595 635 25 0.94 98 2 10 21 20 65 VG A-9 580 620 26 D 94 97 3 9 15 18 100 VG Inv. ex. A-10 570 610 27 0.93 94 6 11 15 18 65 VG A-11 555 600 27 0.93 93 7 13 12 17 55 VG A-12 490 575 29 0.85 99 1 7 12 17 64 VG A-13 640 650 24 0.98 98 2 8 13 24 110 VG Inv. ex. A-14 600 610 25 0.98 100 7 13 20 120 VG Inv. ex. A-15 500 550 26 0.91 100 8 20 16 50 VG A-16 600 635 25 0.94 98 2 8 13 20 60 VG A-17 590 630 25 0.94 98 2 8 21 18 55 VG A-18 590 625 25 0.94 98 2 8 27 18 50 VG B-1 630 630 24 0.93 99 1 8 14 21 100 VG Inv. ex. B-2 630 630 24 0.93 99 1 8 15 22 85 VG Inv. ex. B-3 610 665 25 0.92 99 1 8 22 21 50 VG B-4 625 675 24 0.93 100 12 13 21 65 VG B-5 630 680 24 0.93 100 8 15 25 90 VG Inv. ex. B-6 620 670 24 0.93 100 8 21 24 60 VG B-7 620 670 24 0.93 100 10 23 26 60 VG B-8 515 665 24 0.92 100 10 21 26 65 VG B-9 650 680 24 0.96 97 3 9 14 22 80 VG Inv. ex. B-10 600 640 25 0.94 94 6 12 23 35 55 VG B-11 480 580 27 0.83 98 2 8 12 65 G B-12 625 675 24 0.93 99 1 9 14 22 55 VG B-13 620 670 24 0.93 99 1 9 14 24 50 VG C-1 560 620 27 0.90 98 2 9 12 36 80 VG Inv. ex. C-2 585 600 25 0.98 98 2 9 14 33 70 VG Inv. ex. D-1 605 695 25 0.87 98 2 8 15 30 85 VG Inv. ex. E-1 620 685 24 0.91 98 2 7 14 8 65 VG F-1 570 595 23 0.96 98 2 8 15 52 65 VG Q-1 545 580 28 0.94 100 10 13 44 75 VG H-1 590 720 24 0.82 97 3 10 15 41 65 P I-1 595 715 24 0.83 97 2 1 8 15 42 60 P J-1 615 690 24 0.89 96 6 8 22 33 55 VG K-1 605 720 24 0.84 98 2 7 21 6 60 P L-1 625 680 26 0.92 98 2 9 14 38 80 VG Inv. ex. M-1 665 700 24 0.95 98 2 8 14 37 40 G N-1 595 640 25 0.93 98 2 9 12 33 75 G Inv. ex. 0-1 600 640 25 0.94 98 2 8 13 34 45 P P-1 570 620 27 0.96 98 2 10 13 48 95 VG Q-1 540 595 28 0.91 98 2 8 12 43 110 VG R-1 720 780 21 0.92 98 2 9 21 37 45 VG S-1 615 640 26 0.96 98 2 8 13 56 90 VG T-1 680 720 23 0.94 97 2 8 22 22 65 VG U-1 655 700 24 0.94 98 2 8 21 34 60 VG V-1 665 700 24 0.95 98 2 8 15 36 80 VG Inv. ex. W-1 625 675 24 0.93 98 2 7 14 34 90 VG Inv. ex. X-1 620 670 24 0.93 100 8 15 34 100 VG Inv. ex. Y-1 630 680 24 0.93 100 7 15 35 90 VG Inv. ex. Z-1 650 700 24 0.93 100 8 15 36 100 VG Inv. ex. AA-1 555 635 26 0.87 100 8 13 24 100 G Inv. ex. AB-1 525 630 25 0.83 98 2 8 11 42 65 G AC-1 555 580 28 0.96 100 7 11 41 120 VG d_((F,B)): Average grain size of ferrite and bainite (μm) d_(MCN): Average particle diameter of incoherent alloy carbonitrides ΔHV: HAZ softening of arc weld zone (HV) vE₋₄₀: Charpy impact energy absorption at −40° C. (J/cm²)

Next, the steel sheet was evaluated for strength properties, low temperature impact energy absorption, and bendability.

The steel sheets were evaluated for strength properties by the following method. First, the test material was worked to a No. 5 test piece described in JIS Z 2201. Further, this No. 5 test piece was subjected to a tensile test in accordance with the method described in JIS Z 2241 and the maximum tensile strength (TS), yield strength (YS), and elongation (EI) were found.

The low temperature impact energy absorption was evaluated by a Charpy impact test. Based on JIS Z 2202, a thickness 3 mm 2 mmV-notch test piece was prepared. The test piece was cooled to −40° C., then a Charpy impact test was performed and the impact energy absorption (J/cm²) was measured.

The bending test was performed by the V-block method of JIS Z 224 (bending angle: 90°). The thickness of the test piece was “t”. The limit bending radius r_(lim) with no cracks was measured.

The above measurement results are shown in Table 3. Note that, as explained above, in Table 3, “vE⁻⁴⁰” is the Charpy impact absorption value (J/cm²), while “r_(lim)/t” is the value of the limit bending radius r_(lim) divided by the sheet thickness. An r_(lim)/t of 0.5 or less is ranked as “VG “(very good), over 0.5 to 1.0 or less in range is ranked as “G” (good), and over 1.0 is ranked as “P” (poor).

The Steel A-2 had a slab heating temperature outside of the suitable range, so is a comparative example where then tensile strength was less than 600 MPa and the low temperature impact energy absorption was low.

The Steels A-3 to A-4 and the Steels B-3 to B-4 had rough rolling finish temperatures outside of the suitable range, so are comparative examples where the low temperature impact energy absorptions were low.

The Steel A-6 and the Steel B-3 had times from the end of rough rolling to the start of finish rolling outside of the suitable range, so are comparative examples where the low temperature impact energy absorptions were low.

The Steels A-7 to A-8, the Steel A-10, and the Steels B-6 to B-8 had conditions of finish rolling and cooling conditions after finish rolling outside of the suitable range, so are comparative examples where the low temperature impact energy absorptions were low.

The Steel A-11 and the Steel B-10 had water cooling finish temperatures after finish rolling and coiling temperatures of the hot rolled steel sheets outside of the suitable range, so are comparative examples where the low temperature impact energy absorptions were low.

The Steel A-12 and the Steel B-11 had coiling temperatures of the hot rolled steel sheets outside of the suitable range, so are comparative examples where the tensile strengths were less than 600 MPa and the low temperature impact energy absorptions were low.

The Steel A-15 had an annealing temperature of the Ac3 temperature or more, so is a comparative example where the low temperature impact energy absorption was low.

The Steels F-1, Q-1, S-1, AB-1, and AC-1 had values of amounts of Mn, amounts of Ti, and amounts of Nb outside of the suitable range, so are comparative examples where the amounts of softening of the HAZ were large. Among these, the Steels F-1, Q-1, and AC-1 had tensile strengths of less than 600 MPa.

The Steel G-1 had an amount of C outside of the suitable range, so is a comparative example where the strength was less than 600 MPa and the amount of softening of the HAZ was large.

The Steels H-1, I-1, K-1, and AB-1 had amounts of C, amounts of Si, and amounts of Mn outside the suitable ranges, so are comparative examples where martensite or retained austenite was present, the low temperature impact energy absorption was low, and further the bendability was poor. The Steel J-1 had an amount of Mn outside of the suitable range, so is a comparative example where pearlite was present and the low temperature impact energy absorption was low.

The Steels M-1 and 0-1 had amounts of S and amounts of P which were excessive, so are comparative examples where the low temperature impact energy absorptions were low.

The Steels E-1, R-1, T-1, and U-1 had amounts of Ti, amounts of Nb, and amounts of N outside the suitable ranges, so are comparative examples where coarse alloy carbonitrides were present and the low temperature impact energy absorptions were low.

The Steel P-1 had an excessive amount of Al, so is a comparative example with softening of the HAZ.

As opposed to this, the invention examples all had yield ratios of 0.85 or more, maximum tensile strengths of 600 MPa or more, and excellent low temperature impact energy absorption and HAZ softening resistance. 

1. High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less, a balance of Fe and unavoidable impurities, where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, and having a metal structure which comprises an area percentage of pearlite of 5% or less, a total area percentage of martensite and retained austenite of 0.5% or less, and a balance of one or both of ferrite and bainite, having an average grain size of ferrite and bainite of 10 μm or less, having an average grain size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, having a yield ratio of 0.85 or more, and having a maximum tensile strength of 600 MPa or more.
 2. The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to claim 1, characterized by further comprising, by mass %, V: 0.01 to 0.12%.
 3. The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to claim 1 or 2, characterized by further comprising, by mass%, one or more of Cr, Cu, Ni, and Mo in a total of 0.02 to 2.0%.
 4. The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of claims 1 to 3, characterized by further comprising, by mass%, B: 0.0003 to 0.005%.
 5. The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of claims 1 to 4, characterized by further comprising, by mass%, one or more of Ca, Mg, La, and Ce in a total of 0.0003 to 0.01%.
 6. High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized that the high yield ratio hot rolled steel sheet according to any one of claims 1 to 5 is plated or alloy plated on a surface.
 7. A method of production of high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, heating a steel slab having a composition according to any one of claims 1 to 5 to 1150° C. or more, rough rolling the heated steel slab, finishing the rough rolling at temperature between 1000° C. and 1080° C., wherein a maximum rolling interval in the rough rolling which is performed at 1150° C. or less is 45 sec or less, after the rough rolling, holding the steel slab for a holding time t1 (sec) which satisfies the following formula (1), then starting finish rolling, performing finish rolling with a final rolling temperature Tf which satisfies the following formula (2) so as to obtain a steel sheet, starting water cooling of the steel sheet within 3 seconds after the finish rolling, then cooling the steel sheet to temperature of 700° C. or less at a lowest cooling rate of 8° C./sec or more, and coiling the steel sheet at temperature between 530° C. and 650° C. 1000×([% Ti]+[% Nb])>t1   formula (1) Tf>830+400([% Ti]+[% Nb])   formula (2)
 8. The method of production of high yield ratio hot rolled steel sheet according to claim 7 characterized in that a final rolling temperature Tf satisfies the following formula (3). Tf>830+800([% Ti]+[% Nb])   formula (3)
 9. A method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, pickling the hot rolled steel sheet which was obtained by the method of production according to claim 7 or 8, heating the steel sheet at an Ac3 temperature or less, then dipping the steel sheet in a plating bath to plate the surface of the steel sheet.
 10. The method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to claim 9 characterized by further comprising alloying the plated steel sheet after said plating. 